The maleic anhydride is produced by an oxidation reaction in gaseous phase and is obtained as a diluted and raw, gaseous form.
A first recovery method consists in the water absorption of the maleic anhydride, which reacts with water to form maleic acid.
After the water separation by distillation and the dehydration of the maleic acid, maleic anhydride is recovered.
This recovery type involves, however, a high energy consumption, product loss owing to conversion of maleic acid to fumaric acid, high amounts of pollutants in the effluents and frequent plant stops for washings.
Alternatively, for some time the recovery of maleic anhydride by absorption in an organic solvent has been proposed.
To this process, proposed in the U.S. Pat. No. 2,942,005 describing the absorption of maleic anhydride and phthalic anhydride from reaction mixtures, several operating improvements have been brought.
In the U.S. Pat. No. 3,891,680, the gaseous reaction mixture (from which maleic anhydride is to be recovered) is contacted with an organic liquid phase.
A similar process is disclosed in the U.S. Pat. No. 4,118,403, in which the maleic anhydride is recovered by absorption in an organic liquid to which phthalic anhydride is added.
Finally, the U.S. Pat. No. 5,969,687 discloses the maleic anhydride recovery by means of an organic solvent and, in particular, includes a stripping step of the enriched solvent with a gas of low humidity contents to remove water and volatile substances.
In all the procedures based on the use of an organic absorption solvent, the enriched solvent is passed through a separator under vacuum, in which the raw maleic anhydride is condensed overhead and the depleted solvent leaves from the bottom for being recycled to the absorption phase.
As already mentioned, this fractionating step is common to all solvent recovery processes and leads to produce raw maleic anhydride of a 97-99% purity.
The main residual impurities consist of:
(i) a small amount of solvent
(ii) a certain amount of organic by-products formed in the oxidation reactor and absorbed together with the maleic arthydride in the absorber, and
(iii) water, either free or combined with maleic anhydride.
Typical examples of light organic impurities are acrylic acid and traces of carbonyl compounds.
If the solvent is an ester of a light alcohol (e.g. dibutylphthalate) and the separation between maleic arthydride and solvent is conducted under severe conditions, the alcohol that comes out from the decomposition of the organic solvent might also be present as an impurity in the raw maleic anhydride, typically in the monoester form. Even though the raw maleic anhydride purification can be carried out both in batch and continuous processes, the batch process has some major disadvantages, as for example, the discontinuous steam demand and the necessity of bringing considerable amounts of maleic anhydride to boil, with a consequent, increased risk of exothermal reactions of the maleic anhydride.
The continuous process appears to be preferable and, according to a classical scheme, entails a first fractionating column, within which the light substances are removed, and a second column from the bottom of which the solvent containing a small quantity of maleic anhydride is drawn, that solvent being sent to recycling, whilst the recovered maleic arthydride is drawn in liquid form from the overhead section. However, also the continuous process faces problems and drawbacks not yet solved.
The first of these relates to the drawing of overhead products from the first column where the light substances are separated, including acrylic acid.
The fraction drawn from overhead, and that is sent to a condenser for recycling in liquid phase to the column head, contains acrylic acid in a high concentration, which tends to polymerize and thus to foul the condenser and the column uppermost part in a not negligeable extent.
The second problem connected to the traditional process is represented by water and light or volatile substances present in-the second purification column, where the maleic anhydride is recovered in the upper part of the column itself.
This water might originate from atmospheric humidity infiltrating in the vacuum operating system, or from the decomposition of maleic acid present in the raw maleic anhydride. Water may also be possibly produced by losses from condensers or boilers, due to leaking tube-to-tube sheet joints.
Since such water is being absorbed into the maleic anhydride condensed overhead the second purification column and refluxed overhead the column itself, an unwanted concentration increase of water, that is, of maleic acid, in the maleic anhydride obtained in this column, occurs.
Purpose of the present invention is to substantially eliminate such problems and drawbacks.